参考《疯狂android讲义》第2版 2.1节P48，对应CustomViewDemo.zip。

若在开发过程中，发现现有的view均不能满足需要，可以自定义一个view。

自定义一个view 的关键在于重写view类的几个核心方法，如onDraw， onTouchEvent等。

自定义view类：

DrawBall.java

package com.ljh.customviewdemo.ui;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.util.AttributeSet;import android.view.MotionEvent;import android.view.View;public class DrawBall extends View {	private Paint p = new Paint();	private float currentX = 40;	private float currentY = 50;	public DrawBall(Context context) {		super(context);	}	public DrawBall(Context context, AttributeSet set) {		super(context, set);	}	@Override	protected void onDraw(Canvas canvas) {		super.onDraw(canvas);		p.setColor(Color.RED);		canvas.drawCircle(currentX, currentY, 15, p);	}	@Override	public boolean onTouchEvent(MotionEvent event) {		currentX = event.getX();		currentY = event.getY();		/*		 * Invalidate the whole view. If the view is visible,		 * onDraw(android.graphics.Canvas) will be called at some point in the		 * future. This must be called from a UI thread. To call from a non-UI		 * thread, call postInvalidate().		 */		invalidate();		return true;	}}

Activity类：MainAtivity.java

package com.ljh.customviewdemo;import android.os.Bundle;import android.app.Activity;import android.view.Menu;public class MainActivity extends Activity {	@Override	protected void onCreate(Bundle savedInstanceState) {		super.onCreate(savedInstanceState);		setContentView(R.layout.activity_main);	}	@Override	public boolean onCreateOptionsMenu(Menu menu) {		// Inflate the menu; this adds items to the action bar if it is present.		getMenuInflater().inflate(R.menu.main, menu);		return true;	}}

布局文件：activity_main.xml

注意，activity中包括多个view。一个activity对应一个布局文件，布局文件中包括多个view，这些view可以是android定义好的，或者自定义的。